A tandem printer using electrophotography utilizes, for example, yellow (Y), magenta (M), cyan (C), and black (K) imaging units for printing process in which the recording heads in the imaging units emit light in accordance with print data on the peripheral surfaces of photoconductor drums for exposure to form toner images and the toner images are transferred to a printing medium.
FIG. 10 is an illustration for explaining a prior art control, showing a time chart of controlling yellow (Y), magenta (M), cyan (C), and black (K) stations driving Y, M, C, and K imaging units. As shown in the figure, in the Y, M, C, and K stations, load signals are simultaneously output, individual color print data are supplied, and strobe signals are simultaneously output to drive the individual recording heads.
Therefore, as shown in the figure, the recording heads are equal in power consumption. For example, a current of 500 mA flows through each recording head at the time of not printing and a current of 800 mA flows through the same at the time of printing. In all four imaging units, a total current of 2000 mA flows through the recording heads at the time of not printing and a total current of 3200 mA flows through the same at the time of printing.
A prior art for preventing excessive power consumption is disclosed in Patent Literature 1, in which, for example, the cyan, magenta, yellow, and black LED heads are turned on in sequence to prevent excessive power consumption.
Patent Literature 2 discloses an invention in which the LED array chips are shifted in the sub-scan direction for preventing failure of printed data to be printed on a line because of the LED array chips driven in a time division manner.    [Patent Literature 1] Unexamined Japanese Patent Application KOKAI Publication No. H7-199582    [Patent Literature 2] Unexamined Japanese Patent Application KOKAI Publication No. H7-329352
However, sustained earth environment is called for worldwide and the global warming conference is playing a pivotal role for realizing regulations on emission of greenhouse gasses such as CO2 (carbon dioxide). Under such circumstances, criteria for complying with the “International Energy Star Program” of the ECCJ (Energy Conservation Center, Japan) are set on electric appliances such as printers, facsimiles, copiers, personal computers (PCs), and displays.
The above-described prior art printers are designed to supply a large current to the recording heads at the time of printing, requiring large power consumption. Then, their design does not comply with the above “International Energy Star Program.”
The present invention provides a printer reducing power consumption and complying with the international criteria ensuring sustained earth environment.